Porphyrin



Porphyrins are a macrocyclic compounds containing four pyrole rings. Can you identify them in the scene to the right? (initial scene ) The four pyrole rings are connected together by four methenyl groups. Porphyrins are distinguished from each other by the substituents attached to the pyrole rings. Can you identify the pyrole substituents on this porphyrin (protoporphyrin IX)? They are methyl, vinyl and propioic acid groups. An important function of porphyrins is to form hemes by complexing Fe2+ or Fe3+ with the lone pairs of electrons on the four nitrogens. Heme displayed as spacefill. Since Fe2+ and Fe3+ prefer to complex with six ligands, there are two positions open for nucleophilic groups to bond whenever the heme associates with a protein. Both of these groups may be residues of a protein, or one of them may be a small substrate molecule. In one of these two ways the heme becomes covalently connected to the protein, and therefore can be called a prosthetic group. Residues that are commonly found as the fifth or sixth ligands are His, Cys and Met. Examples of the fifth ligand being His and the sixth ligand being a small molecule are oxymyoglobin and oxyhemoglobin.